Numerical Simulation Analysis on Ozone Source Apportionment in the Yinchuan Metropolitan Area in Summer

Under the current situation of combined pollution due to ozone and particulate matter in China, regional ozone pollution is prominent in Ningxia, northwestern China, in summer. Thus, the regional air quality modeling system RAMS-CMAQ (Regional Atmospheric Modeling System and Models-3 Community Multi-scale Air Quality) was applied to investigate ozone pollution in the Yinchuan metropolitan area (Yinchuan, Shizuishan, and Wuzhong) in June 2019. The contribution of ozone regional and vertical transmission was quantitatively discussed to provide scientific evidence for effective control of local ozone pollution. The results show that O₃ concentration was high in the north and low in the middle of the area, which was not completely consistent with the distribution characteristics of ozone precursors, indicating a nonlinear effect of O₃ and a potential contribution of background O₃ transmission. In the horizontal direction, the contribution of local emission sources to near-surface O₃ was mostly between 20%～30%, except in Lingwu, where it reached more than 40%, and the contribution of external transmission was more than 30%. In Yinchuan, the local contribution was relatively weak, and a transport effect occurred between adjacent areas, weakening the local contribution to some certain extent. Moreover, O₃ was obviously transported from south to north in summer because of overall southerly winds. In the vertical direction, the variation in O₃ concentration was basically similar to the suburban, urban, and industrial representative sites, while the contribution of each process differed. At night, O₃ photochemical loss was the largest at industrial sites, followed by urban and suburban sites, and was the strongest near the ground at all sites. During the day, O₃ was mainly photochemically generated in the upper and middle regions of the boundary layer at urban and industrial sites. Then, together with a remnant in the nocturnal residual layer, O₃ was transported vertically to the ground and horizontally around. Meanwhile, some O₃ in the free atmosphere was entrained down into the boundary layer. Conversely, at suburban sites, photochemical ozone generation occurred throughout the boundary layer and was the strongest near the ground, while the transport effect was relatively weak.